Add Python function to extract particles scraped during the last step.

[RemiLehe]

This adds a function that extracts the particles that were scraped at the current timestep.

This is useful in callback functions, where we often want to re-inject particles that hit the boundary, and therefore need to select the ones that were scraped at the current timestep.

This also avoids calling clear_buffer, which potentially interferes with the BoundaryScrapingDiagnostic

[RevathiJambunathan]

Thank you @RemiLehe for this PR!!!
I tested it and it works!

[dpgrote]

This looks good, thanks!
A minor change would be to have the level default to 0 which is likely the most common case.